1. Field of the Invention
The present invention relates to a propulsion unit for a boat. The propulsion unit is provided with a rotatable impeller in a cylindrical pump housing, and includes stationary blades, disposed on the downstream side of the impeller, for guiding a fluid stream in an axial direction of the pump housing.
2. Description of the Background Art
It is well known it provide a propulsion unit for a boat with an impeller in a pump housing, where the propulsion unit includes stationary blades on the downstream side of the impeller for guiding a fluid stream in an axial direction of the pump housing, and an intake for extracting cooling water. Such a propulsion unit is disclosed, for example, in Japanese published patent document JP-A No. 246298/2003 (FIGS. 1 and 2).
FIG. 8 of the present drawings is a reproduction of FIG. 1 taken from JP-A No. 246298/2003 for comparative purposes, and is a sectional view showing the configuration of a water propulsion unit of a small-sized boat. In the known water propulsion unit P of FIG. 8, the unit P is provided with a front casing 121Cf and a rear casing 121Cr fitted to the front casing 121Cf. Also in the known water propulsion unit P of FIG. 8, a joint gap 102 is provided to a part in which these casings 121Cf, 121Cr are fitted, and a cooling water 101 intake is provided in the joint gap.
The detailed structure of the prior art cooling water intake will be described below.
FIG. 9 is a reproduction of FIG. 2a from JP-A No. 246298/2003, and shows a configuration in which the two casing components 121Cf, 121Cr of the water propulsion unit are fitted, and shows that cooling water is taken in by providing the joint gap 102 at the location in which the front casing 121Cf and the rear casing 121Cr are fitted, forming ringed space 103 in the joint gap 102. The cooling water intake 1 (hereinafter only called the intake 1) is attached to the space 103.
Incidentally, this type of boat may be towed by another boat, except while running normally. However, when the boat according to this known design is towed while its engine is stopped, a considerable quantity of water may enter a water muffler from the intake 1, even though the impeller is stopped, as a result of the boat being pulled through the water. When water exceeding a defined quantity accumulates in the water muffler, startability of the engine is negatively affected.
Therefore, a propulsion unit provided with an intake in which minimal cooling water enters when a boat is towed, and which can take in sufficient cooling water when an impeller is turned, is desired.